Two classes of polyribosomes are present in the cytoplasm of most eukaryotic cells; one bound to the membranes of the endoplasmic reticulum and the other free in the cell sap. Their protein products --in regard to final destination -- fall into four categories: a) exported from the cell, b) segregated into membrane-bound organelles, c) incorporated into cell membranes, d) retained in the cell sap. We wish to elucidate the mechanisms which determine the subcellular distribution of proteins synthesized by membrane-bound ribosomes. Structural aspects of the interrelationships among nascent polypeptides, ribosomes and membranes at the ribosome-membrane junction will be examined by biochemical and electron microscopic techniques. The molecular components involved in the ribosome-membrane interaction will be identified and factors which regulate this association will be characterized. Experiments are also proposed designed to investigate how certain classes of mRNA are selected for translation in bound polysomes. These involve a determination of the amino-terminal sequence of in vitro translation products of mRNA from bound polysomes. We postulate that sequences in nascent polypeptides, which are removed during processing may contain signals for binding of ribosomes to microsomal membranes and for further discrimination between products destined to different intracellular or extracellular locations. We will attempt to reconstruct functional rough microsomes to study the biosynthesis and pathway of delivery of secretory and membrane proteins such as albumin, cytochrome P450 and the envelope glycoprotein of vesicular stomatitis virus which is a protein destined to the plasma membrane.